1. Field of the Invention
The present invention relates to implantable medical devices and methods, and more particularly to a pacing system and method for recording and reporting the occurrence of pacing events associated with the operation of an implantable pacemaker in combination with the respective rates of occurrence of such events.
One of the most common types of implantable medical devices in use today is the implantable pacemaker. Modern pacemakers are small, battery-powered electronic devices that monitor the activity of the heart to determine when the heart is naturally beating, and provide stimulation pulses to the heart when the heart is not naturally beating, thereby maintaining a prescribed heart rhythm or rate. Advantageously, a pacemaker may be implanted in a patient, and coupled to the patient's heart via appropriate pacemaker leads that are also implanted. By implanting the pacemaker and leads, the pacemaker becomes an integral part of the patient, and the patient is able to maintain a substantially normal life style without the bother and worry that typically accompany the use of external (non-implanted), life-sustaining medical devices.
Nearly all implantable pacemakers in use today, as well as similar implantable medical devices, can be configured by the attending physician in the physician's office. The process of configuring a pacemaker is commonly referred to as "programming." The programming process uses noninvasive telemetry to customize the operation of the pacemaker to fit the individual needs of the patient. Customization is achieved by adjusting a set of "pacemaker parameters" to values that cause the pacemaker to work in an optimum way for the particular patient within whom the device has been implanted.
Disadvantageously, as the complexity of new implantable devices has evolved over the past several years, it has become increasingly difficult for the attending physician, or other medical personnel, to determine how the pacemaker should be programmed in order to provide the most effective therapy for a given patient. This difficulty is particularly manifest with recent-generation pacemakers that tend to be more automatic and autonomous than earlier-generation pacemakers, which recent-generation pacemakers are controlled by input signals received from a multiplicity of internal sensors. For example, recent "rate-responsive" pacemakers provide stimulation pulses to a patient's heart, as needed, based on the input signals received from one or more physiological or other sensors that attempt to predict just how fast the patient's heart should beat in order to meet the patient's physiological needs.
A significant factor that makes the optimum programming of recent-generation pacemakers more difficult is the variation in each of the sensor inputs from patient to patient. Such variation is caused by numerous factors, including the patient's physical structure, the implant site, the particular disease or malady the patient has and its progression within the patient's heart or other body tissue, the drugs being taken by the patient to treat his or her condition, etc. Thus, to appropriately program the pacemaker for a given patient, the physician must anticipate how the pacemaker will operate given all of these variables, and given all the environments and activities that the patient is expected to encounter. Programming a modern pacemaker may thus comprise an extremely formidable task, for which task there is a critical need for programming aids to assist the physician in anticipating the pacemaker response for each particular patient.
It is known in the art to use programming aids and devices with implantable pacemakers to facilitate the physician's understanding of the pacemaker's programmed operation as its interacts with the patient's natural cardiac activity. For many years, the primary programming aid and source of diagnostic data for use in analyzing the operation of an implanted pacemaker has been the surface electrocardiogram (ECG), in which both pacemaker and heart activity are blended. From the ECG, the activity of the heart--including the contraction of the atria, the contraction of the ventricles, and the timing therebetween--could be displayed. From the pacemaker, the activity of the pacemaker--including when a heart contraction was sensed and when a stimulation pulse was generated--could likewise be monitored through the use of marker signals telemetered from the pacemaker to a remote (non-implanted) receiver, where such signals were processed and displayed as marks on the ECG waveform.
In recent years, specific programming devices have been developed that not only allow the pacemaker parameters to be noninvasively set to desired values, but that also allow the operation of the pacemaker and the heart to be monitored without having to obtain a surface ECG. Such is accomplished by transmitting an intracardiac ECG signal, either alone or in combination with marker signals. See e.g., U.S. Pat. Nos. 4,559,947; 4,596,255; 4,791,936; and 4,809,697.
Unfortunately, while such prior art programming devices have done much to facilitate communications with and analysis of implantable programmable pacemakers, they all suffer from one major drawback--they are limited to real-time data analysis. This is true even though some provide the capability of capturing a short segment, e.g., 30 seconds, of the intracardiac ECG signal, which intracardiac ECG signal, once captured, can advantageously be expanded, compressed, or otherwise processed in a desired manner in order to better examine it. Unfortunately, in order to properly assess some types of problems that may develop for a given patient having an implanted pacemaker, it is frequently necessary to examine the intracardiac signal, or at least the main components thereof, over a much longer period of time, e.g., days, weeks, or months. What is needed, therefore, is not only a programming device that facilitates the physician's ability to understand the interaction of the implanted device with the patient and to evaluate active clinical problems, but also a device that allows the physician to assess the performance of the system over an extended period of time, e.g., on the order of days, weeks, or months.
The commonly used solution to the above-described problem is to use a Holter Monitor, or equivalent external device. A Holter Monitor is essentially a recorder that is carried by the patient. The Holter Monitor senses the surface ECG signal and records it. Thus, after the data-collection period during which the Holter Monitor is used (usually 24 hours), the number of specific cardiac events that occurred, e.g., the number of ventricular contractions, may be determined.
Disadvantageously, Holter Monitors, and equivalent devices, are external devices that must be carried by the patient continuously throughout the monitoring period. Such carrying can be a nuisance and a bother to the patient. Further, such externally-carried recording devices suffer from numerous limitations. One of the main limitations is their inability to consistently identify the high-frequency pacing stimulus generated by the pacemaker. This is particularly the case when the pacemaker generates a bipolar pacing pulse, which pacing pulse is of relatively low amplitude and difficult to detect (compared to a unipolar pacing pulse). Thus, although the contraction of the heart may be recordable, there is no way of easily determining whether the contraction was a natural contraction or a paced contraction. Further, if a pacing pulse is generated and the heart does not respond to it (i.e., if there is a lack of "capture"), such event is not detectable.
Another limitation of Holter Monitors, and equivalent externally-carried recording devices, is that they have no way of monitoring, and hence recording, the internal state of the implanted pacemaker. A knowledge of the internal state of a pacemaker would be invaluable in determining the actual pacemaker behavior. Advantageously, the implanted pacemaker "knows" exactly when it paces and senses on each channel and how it is responding to every input signal. Thus, what is needed is an implanted pacemaker that is equipped to track and report its behavior over time; and, upon command, provide such information to an attending physician.
There are at least two prior art devices known to the inventors that attempt to address the above needs. On such device is the Cosmos.TM. 283-01 pacemaker marketed by Intermedics, Inc. of Angleton, Tex. The Cosmos 283-01, using a feature termed "Diagnostic Data.TM.," counts the number of specific cardiac events that occur during the monitoring period. See, Sanders et al., "Data Storage and Retrieval by Implantable Pacemakers for Diagnostic Purposes," PACE. Vol. 7, pp. 1228-33 (1984); and Levine, P. A., "Diagnostic Data: An Aid to the Follow-Up and Assessment of the Pacing System," Journal of Electrophysiology, Vol. 1, pp. 144-53 (1987). The other device is the CHORUS.TM. 6033 pacemaker marketed by ELA Medical, of Montrouge, France. Both of these devices, and similar devices described in the literature, see, e.g., U.S. Pat. No. 4,513,743 (van Arragon et al.), are thus able to determine the total number of occurrences of a selected cardiac event, e.g., the number of times that an atrial pulse is issued; or the number of times that an atrial-to-ventricular (AV) interval has a delay that falls within defined time limits. Disadvantageously, however, such devices do not record the frequency of occurrence of the cardiac events that are counted. This limitation prevents many necessary evaluations from being performed, such as the determination of sensor responsiveness and an assessment of chronotropic competence. What is needed, therefore, is an implantable medical device that not only detects and records the occurrence of specified cardiac events, and in particular pacemaker events or states, but that also determines and records the frequency of occurrence, or the rate, associated with each such detected and recorded event.
The present invention advantageously addresses the above and other needs.